1. Field of the Invention
The invention relates to a two-component developer and two-component developing apparatus used for visualizing latent images such as electrostatic latent images in a method for forming images such as electrophotography and electrostatic recording.
2. Description of the Related Art
In electrophotography widely used in image forming apparatuses such as copiers and printers, images are formed in the following manner, utilizing the photoconduction phenomenon with photoconductive materials (e.g., see U.S. Pat. No. 2,297,691, Japanese Examined Patent Publication JP-B2 42-23910 (1967), and Japanese Examined Patent Publication JP-B2 43-24748 (1968)). First, a photosensitive member provided with photosensitive layer containing photoconductive materials is charged with a predetermined potential that may be either positive or negative, and then the charged photosensitive member is exposed to light, corresponding to image information, to form an electrostatic latent image. Then, the formed electrostatic latent images are developed with a developer containing charged microparticles called toner to form a toner image, which is a visible image. The formed toner image is transferred onto a recording material such as a paper sheet as necessary, fixed to the recording material by, for example, heating, pressing, heating and pressing, or exposure to evaporated solvent, so that an image can be obtained.
As the developer used in electrophotography, two types of developers are known, that is, one-component developers, which are made of only toner, and two-component developers, which are made of toner and particles called carriers having magnetism. Among these, the two-component developers are widely used because of, for example, easy control of charging toner. In a two-component developing apparatus in which developing is performed with a two-component developer, the developer is agitated so that toner and carriers are charged with opposite polarities to each other by frictional electrification and supplied onto a developer holding member provided with a magnet therein to form a magnetic brush made of the carriers and the toner, and the magnetic brush is rubbed against the surface of the photosensitive member to develop an electrostatic latent image. Therefore, the carriers in the two-component developer serve to supply charges to the toner by frictional electrification, to convey the toner to the photosensitive member, and the like, and among these, the supply of charges to the toner is particularly important.
In recent years, for an image forming apparatus using electrophotography such as copiers and printers, increasing the speed of image formation and reducing the size are required, regardless of business use or personal use. In order to increase the speed of image formation and reduce the size of the image forming apparatus, it is examined to reduce the size of the developing apparatus by reducing the size of the developer agitating portion and increase of the developing speed. For this reason, for the two-component developers, it is required to charge toner rapidly by frictional electrification with carrier. Furthermore, the image forming apparatus is required to form uniform images over a long time, so that for the two-component developer, it is required that the charging characteristics of the toner and the charging ability of the carrier to the toner are stabilized over a long time.
Furthermore, in order to reduce the size of the developing apparatus, it is effective to reduce the consumption amount of the toner and to reduce the volume of the container of the developer. As the toner, toner in which a colorant or the like is dispersed in a resin having binding properties that is called “binder resin” is used. As a technique for realizing low consumption amount of the toner, it is proposed to improve the coloring ability by increasing the content of the colorant contained in the toner so that an image with a desired image density can be formed with a small amount of the toner. For example, toner in which the concentration of carbon black in the toner is 10% by weight or more is disclosed (e.g., see Japanese Unexamined Patent Publication JP-A 7-77828). However, since carbon black has conductivity, when the concentration of carbon black in the toner is 10% by weight or more, as in the technique disclosed in JP-A 7-77828, the electrical resistance of the toner becomes too low, and the charge amount of the toner becomes too small, so that problems such as image fogging and toner scattering are caused. In order to solve this problem, it is necessary to set the electrical resistance of the carrier to high.
The carrier used in the two-component developer can be classified roughly into coated carrier, which is made of magnetic particles whose surfaces are coated with a coating layer made of, for example, resin, and non-coated carrier, which is made of magnetic particles themselves. Among these, the coated carrier is widely used because of a longer life of the developer and easier control of charging toner than the non-coated carrier. Furthermore, the coated carrier has an advantage that carrier lifting occurs with more difficulty than the non-coated carrier. Herein, “carrier lifting” refers to a phenomenon in which charges having a polarity opposite to that of the charges on the surface of the photosensitive member are introduced to the carrier during development, so that a coulomb force is exerted between the charges on the surface of the photosensitive member and the carrier, and therefore the carrier is attached to the surface of the photosensitive member. When the carrier lifting occurs, the carrier is transferred to a recording material together with the toner, so that critical image defects such as partial transfer defects are caused. It is believed that since the non-coated carrier generally has a lower electrical resistance than that of the coated carrier, the charges with an opposite polarity to the charges on the surface of the photosensitive member tend to be introduced during development, and carrier lifting occurs more easily than in the case of the coated carrier.
Furthermore, since the coated carrier generally has a higher electrical resistance than that of the non-coated carrier as described above, the coated carrier is more effective to solve the problems that are caused when the content of carbon black in the toner is increased. However, when the surface of magnetic particles, which serve as a carrier core material, is coated only with resin, the electrical resistance of the carrier becomes too high, so that the problem that images are degraded because of the edge effect and the phenomenon of accumulation of charges is caused. Herein, the “edge effect” refers to a phenomenon in which when forming an image including a solid image portion with a large area such as a black solid portion, among the solid image portions to which toner is attached, the solid image portion near the boundary with a non-image portion to which toner is not attached is developed with excessive toner, so that the image density in that portion becomes higher than that of the central portion of the solid image portion.
As a technique for solving this problem, it is proposed to disperse conductive particles in the coating layer of the carrier in order to reduce the electrical resistance of the carrier as appropriate and suppress excessive accumulation of charges in the carrier, and to suppress leakage of the charges from the carrier (e.g., see Japanese Unexamined Patent Publication JP-A 58-108549 (1983), Japanese Unexamined Patent Publication JP-A 59-166968 (1984), Japanese Examined Patent Publication JP-B2 1-19584 (1989), and Japanese Unexamined Patent Publication JP-A 6-202381 (1994)).
In this manner, with the coated carrier, desired characteristics can be realized by adding various additives to the coating layer. For example, another conventional technique has proposed to disperse magnetic microparticles in the coating layer in order to prevent the aforementioned carrier lifting (e.g., see Japanese Unexamined Patent Publication JP-A 58-108548 (1983)).
However, in the techniques disclosed in JP-A 58-108549, JP-A59-166968, JP-B21-19584, JP-A6-202381 and JP-A58-108548, the adhesiveness between the coating layer and the carrier core material in the carrier is not taken into consideration, so that the coating layer may be detached and mixed with the toner while agitating the developer. When the coating layer is detached and mixed with the toner, the temperature at which a hot offset phenomenon starts to occur (hereinafter, referred to as “hot offset occurrence temperature) may become lower than the hot offset occurrence temperature when measured only with the toner, depending on the resin constituting the coating layer. Herein, the “hot offset phenomenon” refers to a phenomenon in which when the temperature at which the toner is heated by the fixing member during fixing is too high, the toner melts excessively and is attached to the fixing member.
For example, when resins having a high melting point of, for example, about 250° C. to 350° C. such as silicone resin and fluorocarbon resin (hereinafter, these resins are referred to as “high melting point resins”) are used as the resin constituting the coating layer, and the coating layer is detached and the high melting point resin constituting the detached layer is mixed with the toner, then the hot offset occurrence temperature is lowered. The reason seems as follows. Although the toner is heated to about 170° C. to 220° C. by the fixing member during fixing, the high melting point resin mixed with the toner due to the detachment of the coating layer does not melt at the heating temperature of the toner by the fixing member during fixing, because the melting point thereof is as high as about 250 to 350° C. Therefore, it seems that the high melting point resin serves as if a lubricant during fixing and decreases the melt viscosity of the toner, and therefore the hot offset occurrence temperature is lowered.
In particular, a developing apparatus in which the moving direction of the developer holding member in the portion where the photosensitive member and the developer holding member are opposed to each other, which is the position in which electrostatic latent images formed on the photosensitive member are developed is set to the opposite direction to the moving direction of the photosensitive member (hereinafter, referred to as “counter type developing apparatus”) is used as developing means, the hot offset occurrence temperature tends to be lowered. This is because in the counter type developing apparatus, the amount of the developer that is compressed per unit time is larger in the opposing portion of the photosensitive member and the developer holding member than that of a developing apparatus in which the moving direction of the developer holding member in the developing position is set to the same direction as the moving direction of the photosensitive member, so that the mechanical load applied to the developer is large and the amount of the coating layer detached becomes large.
When the hot offset occurrence temperature is lowered, the heating temperature of the toner by the fixing member has to be set to a lower temperature than the temperature that is suitable to fix the toner on to a recording material. Therefore, the fixing strength for images is decreased, which is a problem. Thus, for the coated carrier, it is required to improve the adhesiveness between the coating layer and the carrier core material.
A conventional technique regarding improvement of the adhesiveness between the coating layer and the carrier core material has proposed to use a substance in which acrylic resin and melamine resin are crosslinked as the material constituting the coating layer (e.g., see Japanese Patent No. 2683624). However, in the technique disclosed in Japanese Patent No. 2683624, the charging characteristics of the toner are not taken into consideration. Therefore, depending on the charging characteristics of the toner, an appropriate charge amount of the toner cannot be obtained, and image density insufficiency, image fogging and toner scattering may result. In particular, when the size of the particles of the toner is reduced, for example, such a size that the volume average particle diameter is about 6 to 9 μm, in order to meet recent requirements of higher definition and higher quality for images, the specific surface area of the toner increases, and the charging ability of the carrier to the toner becomes insufficient. As a result, the charge amount of the toner is reduced, and image fogging and toner scattering tend to occur.